Unfortunately there is no other way for PWA web apps to receive data back from Amber without reading the clipboard.

Once you grant noStrudel permission to read the clipboard I believe it has to be initiated by a user action (ie. click or type) more info here https://developer.mozilla.org/en-US/docs/Web/Security/User_activation
But it could effectively read the clipboard whenever it wanted 

 Yeah that's what I imagined, it's unfortunate but I understand that there is always tradeoffs. Thanks for the detailed reply, I appreciate it!